Have you ever had any meaningful, detailed feedback on your writing? One thing I've found is that once our writing is marginally competent and readable, we rarely get any feedback to get us into the more perfect, publishable realm. I didn't get any detailed feedback until I submitted a draft of my thesis -- then the professor was invested in giving me serious feedback -- entirely related to style, not content. I already had a teaching certificate and an undergraduate English degree.
I can't say I have, but I should. My current program tends to place more emphasis preparing us for the professional world, namely writing memos and reports. So we receive more instruction on content and formatting your ideas for presentation, as opposed to grammatical and stylistic nuances. It's assumed we already know how to write at a graduate level. I haven't received any negative feedback on my writing so far. I suspect this is because my teachers just aren't grading for it. Either way I'm sure my writing can use improvement.